In which one link tucks into the next? (4,5)
I believe the answer is:
food chain
'in which one link tucks into the next?' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'in which one' is the wordplay.
I cannot really understand how this works, but
'in' is found in the answer.
'one' could be 'o' (I've seen this in other clues) and 'o' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for food chain that I've seen before include "hierarchy" , "Natural order of consumption" , "string of sausages?" .)